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Introduction

Since their discovery in 1895, X rays have proved a vital tool of science, making it possible to examine previously hidden worlds ranging from the tiniest of atoms to the most distant galaxies in space. Like light and radio waves, X rays are a type of electromagnetic radiation—oscillating electric and magnetic fields traveling at the speed of light. Their usefulness lies in their ability to penetrate matter.

X rays owe their penetrating power to their relatively short wavelengths and high energy. The wavelengths of X rays range from about 0.05 angstrom to hundreds of angstrom units. (An angstrom is equal to 0.000000004 inch, or 0.00000001 centimeter.) The shorter the wavelength and the higher the energy of an X ray, the deeper it penetrates matter. Visible light, by contrast, has much longer wavelengths and lower energy. When light rays encounter the surface atoms of an opaque substance, they are reflected or absorbed (see Light).

Medical Uses

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One of the earliest applications of X rays was in medicine, where they were used for both diagnosis and therapy. Today X rays are still most widely used in this field. They penetrate soft tissues but are stopped by bones, which absorb them. Thus if a photographic plate that is sensitive to X rays is placed behind a part of the body and an X-ray source is placed in front, X-ray exposure will result in a picture of the internal bones and organs. When the plate, or radiograph, is developed, a negative image is produced: bones and dense tissues show up as light or white regions, while tissues that are easily penetrated by X rays appear dark. Although bones are the most opaque structures in the body, there are many dense tissues, such as cancer tumors, that can also show up unusually light in radiographs. Doctors use these images to diagnose diseases, detect foreign objects in the body, examine dental cavities, and study damaged or broken bones.

In order for X rays to be used for the study of other, less dense tissues of the body, such as the gastrointestinal tract, the tissues must first be made opaque to X rays. Generally, doctors ask patients to drink a liquid mixture containing an opaque material, such as barium, so that the internal contours of the alimentary tract become visible with X rays.

In a similar process—fluoroscopy—doctors use X rays to watch certain internal organs in action. The patient is positioned between an X-ray source and a screen coated with fluorescent material that glows when X rays strike it. In the fluoroscope, a visible image appears when the X rays strike the fluorescent material. The image is brightened by an electronic device called an image intensifier. The image is then displayed on a television monitor that the doctor can observe.

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Conventional X-ray techniques such as these have one major flaw. Structures may be obscured by overlying organs, or soft tissues may be insufficiently delineated for clear viewing. This issue is of particular importance in localizing brain tumors and other damaged sites in the brain. For such applications, a new form of X-ray process was developed, called computed tomography, or CT. It was formerly known as computerized axial tomography, or CAT. In this process the patient is placed inside an X-ray machine, and a narrow beam of X rays sweeps across an area of the body, moving through a slight angle after each X-ray pulse. The resulting series of X-ray images, each taken from a different angle, is recorded electronically and analyzed by a computer. From them, the computer produces a three-dimensional X-ray image.

While CT scanners are expensive machines, generally costing a million dollars or more, they have become invaluable aids in the diagnosis of brain diseases and head injuries. They are capable of displaying subtle differences among materials in the body.

X rays are used extensively in dentistry as well as in medicine. To obtain an X-ray image of the mouth, a piece of photographic film, covered by paper, is held against the inner surface of the teeth. The dentist then switches on the X rays, which travel from a source outside the cheek, through the tissues and dental structures, to the film. Such X rays are used both to detect disease and cavities and to examine the alignment of teeth and their roots.

Considerable care must be taken with X rays, since this radiation is sufficiently energetic to ionize some atoms within the cell—that is, to separate electrons from nuclei—and cause damage. All areas of the patient’s body outside the area intended for the radiograph must be shielded by a material (generally a metal) that absorbs X rays, and X-ray doses should be kept to a minimum. Doctors, dentists, nurses, and technicians must be similarly protected by opaque shields to prevent excessive X-ray damage.

On the other hand, the damaging effects of X rays, like those of other forms of energetic radiation, may be put to medical use. All forms of radiation affect rapidly dividing cells more than slowly developing cells because the genetic material that governs cell division is sensitive to radiation. Therefore, doctors sometimes use concentrated bursts of X rays to kill cancer cells, which divide rapidly, while minimizing damage to the surrounding healthy cells that divide more slowly. Gamma rays, radioactive isotopes, and energetic electrons and nuclei are also used in cancer treatment. (See also Cancer.)

Scientific Uses

Because of their short wavelengths and great penetrating power, X rays can be used to study not only the structure of living organisms, but that of inanimate matter as well. Their chief application as a scientific tool is in crystallography—the study of the structure of crystalline solids. When X-ray waves pass through the regular lattice of a crystal, they interfere with each other and produce a characteristic pattern of spots on a photographic plate. With the aid of a computer, scientists can analyze this pattern and determine how the atoms of the crystal are arranged. X-ray crystallography has been used to study the structure of DNA, the molecule vital to conveying genetic information in all living organisms (see Genetics).

X rays can also be used to identify unknown materials. When an object is bombarded with X rays, its atoms absorb the energy and reemit it with a set of characteristic frequencies. These reemitted X rays are called fluorescence X rays, and by analyzing them scientists can determine the particular elements that make up the material.

Since the 1970s X rays have been used as a powerful probe to study distant astronomical objects. X rays, which can penetrate all mirrors, are difficult to focus in a conventional telescope, but special X-ray telescopes are designed so that incoming X rays strike deeply concave mirrors at small, glancing angles so that they can be brought to a focus on detectors (see Telescope). Extremely hot bodies of gas in space radiate their energy primarily in the form of X rays, as do certain very dense objects such as neutron stars, the collapsed cores of ancient stars. Although these X rays from space are absorbed by the Earth’s atmosphere, X-ray telescopes on board satellites orbiting far above the atmosphere have brought back a wealth of new information.

Industrial and Other Uses

Just as doctors can use X rays to find hidden diseases inside the human body, so manufacturers can use the same radiation to find concealed flaws inside a product. Since X rays can penetrate even the thickest material, small cracks in rockets or other large machines can be detected. X-ray inspection is routinely used to examine metal parts for internal stress defects, which show up as shadows on X-ray photographs.

During the 1980s the use of X rays was introduced in the production of exceedingly small electronic microcircuits. Previously, integrated circuits had been manufactured by coating a silicon wafer with a photosensitive material, then exposing it to a light masked by a special pattern of lines corresponding to the circuit design. The portions of the photosensitive material that were struck by the light could then be removed to create the permanent pattern of lines that made up the finished circuit (see Electronics, “Integrated Circuits”; Microprocessor). As finer and finer lines were required for ever smaller circuits, the wavelength of the light became too long, and X rays with much shorter wavelengths were substituted.

In addition to their uses in medicine, science, and industry, X rays have also been widely employed for security purposes. Millions of travelers have had their hand baggage X-rayed at airport inspection stations, and similar machines are frequently used for customs inspections as well.

With the aid of X-ray inspection, art historians and museum curators have discovered hundreds of art forgeries. X-ray photographs of metal sculptures purported to be from ancient Greece or Egypt, for example, may show internal support structures of a type used only in modern times. Similarly, X rays can reveal older paintings that have been covered up by layers of paint.

Generation, Detection, and Measurement

The principal method by which scientists generate X rays is to allow accelerated electrons to collide with a target of heavy metal foil. The electrons are accelerated in a vacuum under the influence of an electric field. When they collide with the target they produce X rays by two processes.

First, the accelerated electrons collide with the target electrons and excite them to higher energy levels. When, after a time, the target electrons drop back from these excited levels to their initial energy levels, they emit X rays. Second, when the accelerated electrons are slowed by the electrons and nuclei in the target, they themselves emit X rays.

A similar process occurs when any electron is accelerated—that is, changes direction or speed. An accelerated electron emits radiation, and the higher the energy of the electron the higher the energy of the radiation (see Radiation).

By controlling the voltage of the accelerating tube, an X-ray technician or scientist can control the energy of the accelerated electrons and thus the energy and frequency of the emitted X rays. For extremely high-energy X rays, such as those used to penetrate thick metal parts, a linear accelerator (commonly abbreviated linac) may be used.

Linacs repeatedly apply small accelerating voltages to electrons as they move down the tube. Therefore, electrons that began with around one electron volt of energy arrive at their destination with energies of millions of electron volts.

In the mid-1980s a new method of generating X rays was developed—the X-ray laser. These lasers focus the light produced by a conventional laser onto a thin metal wire, heating it intensely to produce a hot plasma, or ionized gas. The atoms of this gas are highly excited and emit X-ray photons, or packets of light (see Light). These photons, in turn, strike other excited atoms, stimulating them to emit more X rays. This cascading effect produces an intense beam of X rays. Such X-ray lasers, once perfected, could be used in chemical studies to produce X-ray holographs (three-dimensional images) of individual molecules during the course of chemical reactions.

The United States Department of Defense is developing another type of X-ray laser as a potential weapon in which the initial source of energy is an exploding hydrogen bomb. This laser would concentrate a fraction of the bomb’s energy into a narrow beam capable of striking targets in space at a distance of thousands of miles. This weapon system is called the Strategic Defensive Initiative. Popularly named “Star Wars,” it was announced by President Ronald Reagan in March 1983.

In nature, X rays are produced deep in space when highly energetic electrons are accelerated as they move through magnetic fields. The excited atoms of hot plasma in interstellar space are another X-ray source (see Plasma and Plasma Physics).

In medical imaging, X rays can be detected with a fluorescent screen/photographic film combination. The fluorescent screen accepts the energy of the X rays and produces a visible image that is permanently recorded by the photographic film. There are a number of detection methods. Ionization counters exploit the tendency of X rays to ionize gases or solids. X rays strip electrons from atoms and leave the electrons free to conduct electricity. By measuring the conductance of a gas or solid with an ionization counter, scientists can determine the amount of ionization and thus the amount of X rays that created the ions. Someionization detectors use semiconductor materials across which a voltage is applied. These are termed solid-state detectors.

A second category of detectors is based on the phenomenon of scintillation. When an atom is excited by an X ray, but emits radiation in the visible range, the process is called scintillation. The resulting photons are amplified in a photomultiplier and then counted by electronic circuits.

In many applications, it is necessary to know not only the intensity of the X rays, but also their energy distribution, or spectrum. For this purpose, X-ray spectrometers are used to differentiate between X rays of varying energy. (See also Spectrum and Spectroscope.)

Discovery

X rays were discovered during the course of research into the effects of high-voltage electricity on gases. In these experiments, a gas was thinned to a partial vacuum in a glass tube called a Crookes tube. Researchers found that, when a current was passed through the gas between electrodes, the gas glowed in a pattern of light and dark areas that changed with changes in voltage and in the degree of vacuum. Sir William Crookes of England and Philipp Lenard and Heinrich Geissler of Germany found that the glow is caused by particles or waves streaming from the cathode, or electron-emitting electrode, to the anode, or electron-collecting electrode, of the tube. (Hence for many years the stream of particles now known to be electrons was called a cathode ray.)

In 1895 Wilhelm Conrad Roentgen of Germany was experimenting with these cathode rays when he noticed that a fluorescent material glowed when a Crookes tube was operated nearby, even though the tube was well masked in cardboard. Materials placed between the Crookes tube and the fluorescent material diminished the glow but did not eliminate it.

Roentgen could not determine how the radiation was carried through space or why it had such penetrating power. For this reason he called the radiation X rays, taking the name from the mathematician’s use of x to denote the unknown quantity in a problem. The formal name given the radiation is Roentgen rays, in honor of the discoverer. (Radiographs are also sometimes called roentgenograms.) (See also Roentgen.) Although scientists have since come to understand the nature of X rays, the original name has persisted in common usage.

In 1916 American physical chemist William D. Coolidge patented the Coolidge tube, which became the prototype of the modern X-ray tube. Earlier X-ray tubes had obtained electrons by ionizing the few molecules of gas left in the tube. The Coolidge tube supplied electrons from a coiled, hot wire of tungsten in the cathode. (Hence the tube is often called a hot-cathode tube.) The tube is capable of producing highly predictable amounts of radiation, and its efficiency was later increased by using a high vacuum.

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